1909] MERWIN AND LYON—SAP PRESSURE IN THE BIRCH 447 
of a few minutes’ duration have almost no effect upon pressure in the 
maple. The Vermont Bulletin records one instance when a wavy 
line given by a recording gauge was probably due to variations in 
sunlight, pressure falling slightly when a cloud obscured the sun. 
Our record of April 18, 1904, shows conclusively that maples may 
respond notably to variations in sunlight. In fig. 1, lines A, B, G 
are pressure curves for maples. At 11 A.M. and at 2:00 and 3:35 
P.M. the irregularities in the curves were observed to be directly 
related to insolation. As to the amount of tension that has been 
observed in maples, our highest records were from a'25°™ tree March 
12, 1902, 75°", and from a 10°™ tree April 5, 1904, 69°". The 
Vermont Bulletin (p. 75) records a pressure equal to 129°" on March 
21, 1898. Pressures exceeding 75°™ are only occasionally observed. 
Negative pressures seldom exceed 20°". 
PART II 
H. E. MERWIN 
Causes of sap pressure variations in the birches 
The studies of Ig06—1908 were carried on in Cambridge, Mass., 
in the hope of getting more data as to the causes of pressure variations 
in birches. 
The character of both the long and the short period oscillations on 
the pressure curve, and the corresponding record of a freely exposed 
blackened-bulb thermometer for several days, are shown in fig. 3. 
Several important relations are to be noted between the two curves. 
During the day there is a close parallelism; at night the pressure curve 
rises regardless of temperature. In other words, maximum pressure 
and maximum insolation occur at about the same time, near the middle 
of the day; but minimum pressure comes near sunset, while minimum 
temperature is nearly 12 hours later, shortly before sunrise. Some 
of the factors in the control of sap pressure are brought out in the 
several experiments and discussions that follow. The details of 
the longer experiments referred to in the general discussion are given 
under a later heading, 
Experiment shows that during the sap season for the birch, all 
the intercommunicating cavities of the roots and stem are kept 
Practically full of sap. One tree (exp. 1) gave the calculated gas 
